The present invention relates to estimating a Doppler shift compensation in a mobile communication system.
In a mobile communication system, signals which are transmitted from mobile stations moving relative to a base station are subject to the well-known Doppler effect, which causes a frequency shift in the frequency received at the base station relative to that which was transmitted by the mobile station. This frequency shift is referred to herein as the Doppler shift. The Doppler shift is dependent upon the speed and direction of movement of the mobile station relative to the base station. Thus, the Doppler effect can provide an increase or a decrease in the frequency, depending on the direction of movement of the mobile station relative to the base station. The magnitude of the Doppler shift is dependent on the speed with which the mobile station is moving relative to the base station.
Another area of frequency offsets arises when the transmitter unit and the receiver unit are wrongly synchronised.
Existing mobile communication installations provide a form of Doppler compensation, in that the frequency detection circuitry within the base station which selects a particular signal on a particular channel can take into account a certain amount of Doppler shift in the signal.
AU 664626 relates to a method and circuit arrangement for compensating for the Doppler shift in a radio signal propagating between a base station and a mobile station when the mobile station approaches and moves past the base station. As a mobile station approaches the base station with a decreasing propagating time delay, the propagating time delay is integrated at intervals to determine how the propagating time delay varies with time. This information is used to effect a change in the radio signal frequency at a particular time to compensate for the sudden Doppler shift as the mobile station moves past the base station. In a TDMA system, the interrogation occurs at intervals equal to an integral number of time frames. In the GSM standard a time frame comprises eight consecutive time slots and a single transmission burst passes between a particular mobile station and base station in any one time frame.
Thus, in this method of Doppler compensation decisions are made in response to past and incoming signals to improve the reception at a future time, i.e. it is a reactive system. It would be desirable to implement a system which can actively compensate for Doppler shifts in an incoming signal in real time.
Moreover, due to fading a signal subject to the Doppler effect may have a variation in amplitude as well as frequency. It would be desirable to estimate the Doppler effect on amplitude as well as frequency.